The present invention pertains to organic electroluminescent displays and methods for making the same.
Organic electroluminescence (EL) has been studied extensively because of its possible applications in discrete light emitting diodes (LED), arrays and displays. Organic materials can potentially replace semiconductors in many LED applications and enable wholly new applications. The ease of organic LED (OLED) fabrication and the continuing development of improved organic materials promise novel and inexpensive OLED display possibilities.
Organic EL at low efficiency was reported many years ago in e.g. Helfrich et al., Physical Review Letters, Vol. 14, No. 7, 1965, pp. 229-231. Recent developments have been spurred by two reports of efficient organic EL: C. W. Tang et al., Applied Physics Letters, Vol. 51, No. 12, 1987, pp. 913-915, and Burroughes et al., Nature, Vol. 347, 1990, pp. 539. Tang used vacuum deposition of molecular compounds to form OLEDs with two organic layers. Burroughes spin coated a polymer, poly(p-phenylenevinylene), to form a single-organic-layer OLED. The advances described by Tang and in subsequent work by N. Greenham et al., Nature, Vol. 365, 1993, pp. 628-630, were achieved mainly through improvements in OLED design derived from the selection of appropriate organic multilayers and electrode metals.
The simplest possible OLED structure, depicted in FIG. 1A, consists of an organic emission layer 10 sandwiched between cathode 11 and anode 12 electrodes which inject electrons (exe2x88x92) and holes (h+), respectively, which meet in the emission layer 10 and recombine producing light. It has been shown (D. D. C. Bradley, Synthetic Metals, Vol. 54, 1993, pp. 401-405, J. Peng et al., Japanese Journal of Applied Physics, Vol. 35, No. 3A, 1996, pp. L317-L319, and I. D. Parker, Journal of Applied Physics, Vol. 75, No. 3, 1994, pp. 1656-1666) that improved performance is achieved if the electrode work functions match the respective molecular orbitals (MO) of the organic layer 10. Such an improved structure is shown in FIG. 1B where optimized electrode materials 13 and 14 reduce the energy barriers to carrier injection into the organic layer 10. Still, single organic layer structures perform poorly because electrons can traverse the organic layer 10 reaching the anode 14, or holes may reach the cathode 13, in either case resulting in current without light, and lower OLED efficiency.
Balanced charge injection is also important. For example, an excellent anode is of limited use if the cathode has a large energy barrier to electron injection. FIG. 2 illustrates a device with a large electron barrier 16 such that few electrons are injected, leaving the holes no option but to recombine non-radiatively in or near the cathode 15. The anode and cathode materials should be evenly matched to their respective MOs to provide balanced charge injection and optimized OLED efficiency.
An improved structure in which the electron and hole transport functions are divided between separate organic layers, an electron transport layer 20 (ETL) and a hole transport layer (HTL) 21, is shown in FIG. 3. In C. W. Tang et al., Journal of Applied Physics, Vol. 65, No. 9, 1989; pp. 3610-3616, it is described that higher carrier mobility is achieved in a two organic layer OLED design, and this led to reduced OLED series resistance enabling equal light output at lower operating voltage. The electrodes 22, 23 can be chosen individually to match to the ETL 20 and HTL 21 MOs, respectively, while recombination occurs at the interface 24 of organic layers 20 and 21, far from either electrode. As electrodes, Tang used a MgAg alloy cathode and transparent Indium-Tin-Oxide (ITO) deposited on a glass substrate as the anode. Egusa et al. in Japanese Journal of Applied Physics, Vol. 33, No. 5A, 1994, pp. 2741-2745 have shown that proper selection of the organic multilayer materials leads to energy barriers blocking both electrons and holes at the organic interface. This is illustrated in FIG. 3 in which electrons are blocked from entering the HTL 21 and holes from entering the ETL 20 by a clever choice of organic materials. This feature reduces quenching near the contacts (as illustrated in FIG. 2) and also promotes a high density of electrons and holes in the small interface volume providing enhanced radiative recombination.
With multilayer device architectures now well understood and widely used, a remaining performance limitation of OLEDs is the electrodes. The main figure of merit for electrode materials is the position of the electrode Fermi energy relative to the relevant organic MO. In some applications it is also desirable for an electrode to be either transparent or highly reflective to assist light extraction. Electrodes must also be chemically inert with respect to the adjacent organic material to provide long term OLED stability.
Much attention has been paid to the cathode, largely because good electron injectors are low work function metals which are. also chemically reactive and oxidize quickly in atmosphere, limiting the OLED reliability and lifetime. Much less attention has been paid to the optimization of the anode contact, since conventional ITO anodes generally outperform the cathode contact leading to an excess of holes. Due to this excess, and the convenience associated with the transparency of ITO, improved anodes have not been as actively sought as improved cathodes.
ITO is by no means an ideal anode, however. ITO is responsible for device degradation as a result of In diffusion into the OLED eventually causing short circuits as identified by G. Sauer et al., Fresenius J. Anal. Chem., pp. 642-646, Vol. 353 (1995). ITO is polycrystalline and its abundance of grain boundaries provides ample pathways for contaminant diffusion into the OLED. Finally, ITO is a reservoir of oxygen which is known to have a detrimental effect on many organic materials (see J. C. Scott, J. H. Kaufman, P. J. Brock, R. DiPietro, J. Salem, and J. A. Goita, J. Appl. Phys., Vol. 79, p. 2745, 1996). Despite all of these problems, ITO anodes are favored because no better transparent electrode material is known and ITO provides adequate stability for many applications.
While conventional OLEDs extract light through the ITO anode, architectures relying on light extraction through a highly transparent cathode (TC) are desirable for transparent OLEDs or OLEDs fabricated on an opaque substrate. Si is an especially desirable OLED substrate because circuits fabricated in the Si wafer can be cheaply integrated with drive circuitry providing display functions. Given the minaturization and outstanding performance of Si circuitry, a high information content OLED/Si display could be inexpensively fabricated on a Si integrated circuit (IC).
The simplest approach incorporating a TC is to deposit a thin, semi-transparent low work function metal layer, e.g. Ca or MgAg, followed by ITO or another transparent, conducting material or materials, e.g. as reported in Bulovic et al., Nature, Vol. 380, No. 10, 1996 p. 29, or in the co-pending PCT patent application PCT/IB96/00557, published on Dec. 11, 1997 (publication number W097/47050). To maximize the efficiency of such a TC OLED, a highly reflective anode which can direct more light out through the TC is desired. Consequently, the low reflectivity of ITO is a disadvantage in TC OLEDs.
Alternatively, for some applications it may be more important to increase the contrast ratio of the OLEDs or display based thereon. In this case, a TC OLED could benefit from a non-reflective, highly absorbing anode. Again the optical characteristics of ITO are a disadvantage.
High work function metals could form highly reflective anodes for TC OLEDs. Some of these metals, e.g. Au, have a larger work function than ITO (5.2 eV vs. 4.7 eV), but lifetime may be compromised because of high diffusivity in organic materials. Like In from ITO, only worse, Au diffuses easily through many organic materials and can eventually short circuit the device.
Efforts have been made to fabricate OLEDs on Si substrates (Parker and Kim, Applied Physics Letters, Vol. 64, No. 14, 1994, pp. 1774-1776). Si, due to its small bandgap and moderate work function, has a large barrier for both electron and hole injection into organic MOs, and therefore performs poorly as an electrode. Parker and Kim improved the situation somewhat by adding a SiO2 interlayer between the Si and OLED. A voltage drop across the SiO2 insulator permitted the Si bands to line up with their organic MO counterpart and electrons or holes from the Si to tunnel through the SiO2 into the organic MO. However, the required voltage drop across the SiO2 raised the OLED turn-on voltage  greater than 10 V, making these OLEDs inefficient. Low voltage OLED/Si designs are desirable not just to improve efficiency, but also to facilitate circuit design since sub-micron Si transistors cannot easily produce drive voltages  greater than 10 V. For anodes, more desirable than a tunneling insulator surface modification like SiO2 is one which raises the Si surface work function thereby lowering the OLED operating voltage.
As can be seen from the above examples and description of the state of the art, electrode materials must be improved to realize OLEDs, and displays based thereon, with superior reliability and efficiency, and to enable novel architectures, such as devices emitting through a TC. In particular, to fabricate an OLED array or display on a Si substrate, an improved anode compatible with Si IC technology is required for optimized TC OLED architectures.
It is an object of the present invention to provide new and improved organic EL devices, arrays and displays based thereon.
It is a further purpose of the present invention to provide new and improved organic EL devices, arrays and displays based thereon optimized for light emission through a transparent cathode electrode with improved efficiency, lower operating voltage, or steeper current/voltage characteristic and increased reliability, stability and lifetime.
It is another object of the present invention to provide new and improved anodes for organic EL devices, arrays and displays fabricated on Si substrates.
It is a further object to provide a method for making the present new and improved organic EL devices, arrays and displays.
The above objects have been accomplished by providing an OLED having a cathode, an anode, and an organic region sandwiched in between, said anode being composed of
a metal layer,
an anode modification layer, and
at least one barrier layer,
said anode being arranged such that said anode modification layer is in contact with said organic region and light is extracted through said cathode.
Any kind of metal is suited as metal layer in connection with the present invention. Examples are Al, Cu, Mo, Ti, Pt, Ir, Ni, Au, Ag, and any alloy thereof, or any metal stack such as Pt on Al and the like.
The inventive approach is specifically designed for the fabrication of OLEDs on top of Si, preferably Si crystalline wafers incorporating pre-processed integrated display circuitry (herein referred to as Si IC). The present invention is designed to modify the existing Si device metallization into a stable OLED anode having good hole injection properties. For OLEDs on top of a Si IC, the metal layer in the present invention is generally the final metallization layer of the Si IC process, which consistent with present Si technology is normally Al, Cu or an alloy thereof. Neither Al, Cu or Al:Cu alloys perform well as OLED anodes, but they do provide excellent visible spectrum reflectivity which increases the amount of light extracted through a TC. The Si IC metallization surface can vary widely in terms of oxide thickness, roughness and surface contaminants depending on numerous factors, including the fabrication process, the time between IC fabrication and OLED deposition, and the environment in which the Si IC was stored and shipped. For reproduceable fabrication of efficient OLEDs the Si metallization anode properties must be improved and effects arising from variations of the initial state of the metal surface must be eliminated.
The inventive approach is also suited for use with pixel and drive circuitry comprising polysilicon or amorphous silicon devices.
The anode modification layer in the present invention is mainly selected for its high work function which provides efficient hole injection into OLEDs. The anode modification layer must form a stable interface with the adjacent organic layer being part of the so-called organic region (e.g. the organic HTL) to insure consistent OLED performance over an extended time period. The anode modification layer can be conductive or insulating, but it should be sufficiently thin that it contributes negligibly both to the OLED series resistance and optical absorption losses. Oxides are well suited as anode modification layers. The thickness of the anode modification layer is preferably between 0.5 nm and 10 nm.
The barrier layer or layers in the present invention isolates the anode modification layer from the metal layer by forming a physical and chemical barrier, while permitting charge to pass freely through its interfaces with the metal layer and anode modification layer. The barrier layer(s) provides a consistent and reproduceable surface for the deposition or formation of the anode modification layer regardless of the metal layer composition or initial state of its surface. The barrier layer(s) can be conductive or insulating, but it (they) should be sufficiently thin that it (they) contributes negligibly to the OLED series resistance. Alternatively, the barrier layer(s) can be highly reflective which avoids absorption losses. The thickness of the barrier layer is preferably between 5 nm-100 nm. Well suited are barrier layers comprising TiN or TiNC, for example.
For formation on a Si wafer, all of the layers comprising the anode must be depositable or formable onto the wafer using processes sufficiently gentle that underlying Si circuitry is undamaged, i.e. at low temperature causing little chemical or physical damage.
In one embodiment of the present invention, a single or multilayer OLED structure having a TC fabricated on a Si substrate incorporates a multilayer anode structure comprising a metal layer, an anode modification layer, and an intermediate barrier layer(s), such that the anode is stable and efficient at hole injection.
The introduction of such an anode into the OLED structure leads to the following advantages:
Low voltage hole injection via both the high work function of the anode modification layer and the free passage of charge from the metal layer, through the barrier layer(s) into the anode modification layer.
Stable OLED operation over an extended time period via the chemical and physical barrier the barrier layer(s) provides between the metal layer and anode modification layer, and the stability of the anode modification layer interface with the adjacent organic HTL.
Efficient light extraction through the TC aided by the high reflectivity and low absorption of the metal layer, barrier layer(s) and anode modification layer stack.